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Fundamentals of Concrete Building Foundations

When it comes to structural problems in homes, you should know that most of the times they are caused by faulty foundation work and yes, as you may already suspected, these repairs can be quite costly. In general, repairs are required because when the foundation was poured, the builder made errors in regards to the soil evaluation, foundation design calculations, and also the size.

This generally results in walls that fail or crack, walls that are unstable, or walls that easily allow water to seep into the property. Other issues include settlement that leads to problems with roofs, chimneys, door openings, window openings and floors.

The bad news is that if your home has already been built and you experience foundation problems, these can cost a lot of money and take a long time to fix.

Types of Concrete Foundations

There are generally 4 types of foundations that builders can consider for building a property and if you want to have a family residence built for instance, then it's going to be constructed on 1 of 4 foundation types:

  1. The first type is called a slab foundation. This type is basically a layer of concrete which stretches over the entire area on which the property is going to be built on. Based on the local conditions, it can be constructed with footings or without them.
    Slab Foundation
    The slabs can be fully flat if there are no frosting issues and they can be placed near or at ground level. If there is a chance that the frost may penetrate beneath the slab's edges and produce an upward swelling of soil, condition known as "heaving of the soil", footings are certainly going to be required.In areas where frost penetration is deep, the slab and footing are laid on gravel bed which has been previously drained very well in order to prevent the collection and freezing of moisture.
  1. The full basement foundation is usually made of at least 4 external walls (concrete) which are the ones that will carry the majority of the property's load. In general, these walls are going to have the same size and configuration as the floor plan and they're going to extend into the earth as deep as required to provide good quality living space below the grade.Fundamentals of Concrete Building FoundationsIt's also important to know that these walls are carried on footings which basically have the task of spreading the load over larger areas so they can remain within the soil's safety load capacity.
  1. The pier foundation is basically a series of separate concrete piers or posts that stretch down to the frost line or they can also rest on gravel beds that have been previously drained properly. The property is going to rest on beams that span the piers.
    Pier Foundation
  1. Lastly, surface foundations are very similar to full basement foundations, but with the difference that the footings don't go deeper than the frost line and the earth between the walls is also not going to be excavated.
    Surface Foundation

Foundation Footings Basics

When it comes to residential construction, loads are generally small enough so that the minimum footing sizes the building codes require are good enough for spreading the loads.

In general, the majority of building codes require that footings are minimum 2x as wide as the walls they carry. In terms of depth, it needs to be the same with to its projection width beyond the wall and in no way less than six inches.

When it comes to sloping ground, it is possible to use stepped footings. The forms for footings which are going to be made below ground level should be constructed of three quarter inch lumber.

On the other hand, if these have to be installed on the excavation floor, they have to withstand the concrete's high pressure and that is why it's best they are made of 2'' material. The good news is that forms made of prefabricated steel can be easily found in most home improvement stores.

In hardpan or clay though, it's recommended that the sides of the excavation are going to be used as forms for footings. The posts supporting the main girders are also going to require the use of footings.

It's very important that the tops of the footings are clear of the floor slab's bottom. In terms of size, they should be a minimum of eight inches in height based on the footing area; they need to also cover a large areas so that the soil underneath will be able to withstand the load the post imposes and of course, they should also be constructed square.

The chimney footings are required to have a large enough size so that they're able to carry the necessary weight. So instead of making use of a girder, using a bearing wall on a continuous footing that has the same size as the wall footing and is formed in the same way is a much better idea.

The video below describes types of the foundations and footings:

Footings and Foundation Wall Reinforcement

Even though reinforcement in the footings is very rarely required when it comes to single family homes, by considering it the building is going to be protected against settlement cracking for a very low cost.

To reinforce the building, the builder is going to place rods with hooked ends across the footing's width in such a way that the reinforcement is supported high enough to leave 3'' of concrete underneath it. The hook's ends need to be at a minimum of one inch from the form's side.

Wall To Footing Ties

Last but not least, by embedding number four re-bars as steel dowels in a vertical position in the footing concrete at a distance of just 1''. Regardless if a keyway has already been provided in the top of the footing, this is considered good practice and is very much recommended. For those who are interested, they should know they can purchase proprietary permanent keyways of asphalt as well.

Wall To Footing Ties

As for the keyways made of wood, they should be coated using a single asphalt layer since this is going to facilitate easy removal after the concrete has hardened, but also offers a film of asphalt in the keyway to which the waterproofing asphalt membrane is going to bound.